In compression/decompression (codec) systems, compression efficiency and video quality are important performance criteria. For example, visual quality is an important aspect of the user experience in many video applications and compression efficiency impacts the amount of memory storage needed to store video files and/or the amount of bandwidth needed to transmit and/or stream video content. For example, a video encoder compresses video information so that more information can be sent over a given bandwidth or stored in a given memory space or the like. The compressed signal or data may then be decoded via a decoder that decodes or decompresses the signal or data for display to a user. In most implementations, higher visual quality with greater compression is desirable.
In some video standards, the transform unit is an important feature for compression efficiency and quality. For example, the high efficiency video coding (HEVC) standard defines coding units (CUs) as picture sub-partitions having variable sizes. HEVC also defines prediction units (PUs) and transform units (TUs) that specify how a given coding unit is to be partitioned for prediction and transform purposes, respectively. After intra or inter prediction, transform operations are applied on residual bocks to generate transform coefficients, which are quantized, scanned, and entropy encoded into a bitstream.
The transform units may thereby allow a coding unit to use a smaller transform size and support transform sizes from, for example, 4×4 to 32×32. Although discussed with respect to HEVC, other coding standards such as VP9 use similar features. An optimal or near optimal selection of transform unit sizes may bring about large subjective and objective improvements in the coding processing. Currently, the selection of transform unit sizes may require brute force rate distortion based transform unit size selections, which is based on multiple pass coding for each available transform unit size. However, the complexity of such brute force selection techniques may not be feasible in many implementations.
It may be advantageous to select transform unit sizes with efficiency and high quality. It is with respect to these and other considerations that the present improvements have been needed. Such improvements may become critical as the desire to compress and transmit video data becomes more widespread.